Mammalian type-C viral reverse transcriptases have been purified by ion exchange and affinity chromatography. Sensitive radioimmunoassays, which can detect reverse transcriptase even in the absence of enzymatic function, have been developed. The viral enzyme has been shown to possess type-specific antigenic determinants, as well as determinants shared by all known mammalian type-C viral reverse transcriptases. Thus, rapid type-specific identification of reverse transcriptases in new viral isolates or in tissues harboring type-C viruses is possible. Reverse transcriptase competition radioimmunoassays have been used to quantitate the enzyme in type-C virus-infected cells. The synthesis and degradation of type-C viral gene products in infected cells is being investigated. These assays should be useful in studying reverse transcriptase expression in mammalian cells possessing endogenous type-C viruses, and in searching for subviral expression of type-C viral components in normal and tumor-bearing tissues of higher primates.